It is known that a range of materials such as foodstuffs, and agricultural products and fossil fuels including coal, may be pulverized in a pulverizing operation performed by a bowl mill. A bowl mill, in one typical configuration, includes a separator body and a grinding table. The grinding table is supported on the top axial end of a shaft for rotation within the separator body. Such a bowl mill also includes a plurality of grinding rolls supported within the separator body, each grinding roll being operable to exert a grinding force on the material to be pulverized which is disposed on the grinding table for effecting the pulverization thereof. The portion of the bowl mill comprising the grinding table and the grinding rolls can be viewed as an upper region of the bowl mill in which the feed of the material to be pulverized, the pulverization of the material, and the classification of the material is performed. In contrast, the portion of the bowl mill below the grinding table can be viewed as a lower region which includes the grinding table shaft and the shaft rotation drive assembly, typically in the form of a gear assembly and a drive motor.
In a negative pressure bowl mill operation, a gas stream typically comprised of heated outside air from a forced draft (FD) fan is drawn into the upper region of the bowl mill, typically by means of an exhauster. The exhauster is typically located downstream of the bowl mill. Under normal design operation, the upper housing is customarily at a pressure of between one and one half (1.5) to one half (0.5) inches of water less than the lower chamber. This airstream in the upper region of the bowl mill acts entrain individual pieces, including particles and larger pieces, of the material fed into the bowl mill and to transport these entrained pieces into engagement with a device for discriminating between those pieces which are sufficiently pulverized to be permitted to exit the bowl mill and those pieces which require further pulverization within the bowl mill; this discriminating device is frequently a rotating vane-type device known as a rotary classifier.
As noted, under normal design operation, the upper housing is customarily at a relatively lower pressure than the lower housing. Due to this normal operating condition, bowl mills designed for negative pressure operation have typically not been provided with an air seal system of the type often provided on mills designed for pressurized mill operation which require such an air seal system to maintain a separate pressurized region between the upper and lower regions. Accordingly, in bowl mills designed for negative pressure operation, the absence of an air seal system or some other seal system between the upper and lower regions effectively dictates that the upper housing pressure must be maintained at a negative pressure and, moreover, this negative pressure is the operating limit of the bowl mill.
The upper housing negative pressure operates as the operating limit because the coal grinding capacity of the bowl mill is restrained by the need to maintain the negative pressure regime although the bowl mill may be otherwise capable of handling an increased feed rate of coal. For example, if it is desired to increase the feed rate of coal, relatively more hot air is needed to dry and transport the additional coal. However, the exhauster typically cannot push this additional heated air, which is commonly provided via the forced draft (FD) fan, through the milling and fuel pipe system. Thus, when the coal feed rate is exceeded, the heated air is no longer moved through the upper housing at a rate sufficient to maintain a negative pressure regime in the upper housing. Instead, the heated air dwells in the upper housing and pressurizes it.
It can be appreciated that such a positive pressure regime in the upper region of the bowl mill will negatively promote the transport or migration of feed material pieces such as, for example, pulverized coal particles, into the lower region of the bowl mill and this is an undesirable consequence in view of the presence of rotating and lubricated parts in the lower region of the bowl mill. These lower region parts such as, for example, the gear drive assembly for driving the grinding table shaft, will suffer deteriorated performance as well as more frequent operational interruptions and breakdowns if beset by the material feed particles. Accordingly, the bowl mill design and operation should preferably be arranged to accommodate the positive pressure regime in the upper region so that the parts of the bowl mill in its lower region can optimally perform.
U.S. Pat. No. 4,441,720 discloses one approach for minimizing the intrusion of feed material particles into the bowl mill lower region. This patent discloses a barrier air chamber through which flows a barrier air at a pressure exceeding the pressure of the carrier gas in the upper region of the bowl mill. The barrier air is connected through a gap with the upper region and flows thereinto to prevent passage of the carrier gas together with coal dust into the lower region of the bowl mill. While this arrangement may demonstrate some efficacy in preventing penetration or migration of feed material particles into the lower region of the bowl mill, it can be appreciated that such an arrangement requires a separate forced air supply means and brings with it the usual maintenance requirements of any forced gas stream arrangement including seal maintenance and maintenance of the gas stream motive means (i.e., a fan motor). Accordingly, there is still a need for an arrangement which prevents or minimizes the penetration or migration of feed material into the lower region of a bowl mill which offers efficacy and reliability without necessitating relatively substantial maintenance and installation efforts.